The present invention relates to protection of point-to-point unidirectional and bi-directional fiber-optic transmission lines, and particularly to replacement of commonly used optical switches with variable ratio couplers for the protection of the fiber-optic transmission lines and up to about 3 dB insertion loss improvements.
Currently, transmission systems employed in the cable television (CATV) industry provide two-way transmission of information (e.g., video, audio, multimedia and/or data) between a head end and a plurality of subscribers. The head end transmits the information destined for individual subscribers in optical format (i.e., downstream information) through one or more fiber optic links to one or more optical nodes. Each node converts the optically-formatted downstream information into electrical signals for distribution, typically via a coaxial cable plant having a tree and branch architecture, to individual subscribers. In addition to receiving the downstream information, each individual subscriber may generate information in the form of voice, video, data, or any combination thereof, destined for the head end. The subscriber-generated information (i.e., upstream information) is aggregated by the coaxial cable plant and passes to the node for conversion into an optical format for transmission to the head end.
CATV service providers and their subscribers are accustomed to high reliability service. One way in which high reliability service is achieved is by providing two optical paths between the head end and each optical node; a first serving as a primary optical path and a second serving as a secondary (or backup) optical path. An optical switch routes the optical information signals from the primary path to the secondary path in the event of an unanticipated failure in the primary path. The optical switches are often located in the head end and the optical nodes.
The optical switches generally employ an opto-mechanical switching component that switches between the primary path and the secondary path based on the electrical voltage that is applied. A portion of the optical signal in the primary and secondary paths is monitored and converted to an electrical voltage. The voltages are monitored and if a threshold condition is violated, indicating a failure in the primary path, the switch is activated so that traffic is transferred to the secondary path. However, the optical switch does not include any arrangement for switching back from the secondary path to the primary path after the primary path has been restored. Rather, an operator or technician must perform a manual power cycle to restart the optical switches in both the head end and the optical node so that the switches return to the primary path. Restoration in this manner can be difficult because the head end and the optical node may be located 50 to 100 kilometers apart. Also, there may be many such optical switches in both the head and the nodes, thus requiring the operator to take proper care to ensure that the correct combination of switches are power cycled so that there is no interference with traffic on the other paths.
The prior art solution to the above difficulty with responsive switching is to use automatic switches that provide protection based on detection of a change in power level in one of the two optical paths.
FIG. 1 illustrates a block diagram of a unidirectional point-to-point prior art transmission system. An optical transmission system 10 is illustrated having a primary path 12 and a secondary path 14 adapted to receive signals transmitted via an optical path that may include, e.g., an Erbium doped fiber amplifier (EDFA) 11. A conventional 3 dB optical coupler 13 couples the transmitted signal to the paths 12, 14. An optical protection switch module 16 having a 1×2 configuration is operatively coupled to both the primary path 12 and the secondary path 14. The optical protection switch module 16 includes a primary port 18 coupled to the primary path 12 and a secondary port 20 coupled to the secondary path 14. The optical protection switch module 16 includes a switch element 22 coupled to both the primary port 18 and the secondary port 20 and configured to switch between one of the two ports 18 and 20 in order to maintain the optical transmission system 10 operable. A primary detector 24 is coupled to the primary port 18 and configured to perform basic power level detection of the primary path 12. A secondary detector 26 is coupled to the secondary port 20 and configured to perform basic power level detection of the secondary path 14. In the event of a break in the primary path 12, the primary detector 24 detects low power in the primary path 12. The switch element 22 of the optical protection switch module 16 actuates and switches from the primary path 12 to the secondary path 14. The optical protection switch module 16 functions to maintain the optical transmission system 10 by switching to the better path based on power level detection.
Although the optical protection switch module 16 is a useful means of protection for the optical transmission system 10, there are losses (e.g., 2 decibels (dB)) associated with the use of the optical protection switch module 16.
A prior art solution for the protection of a bi-directional point-to-point transmission system is illustrated in FIG. 2. A block diagram of a bi-directional point-to-point optical transmission system 30 is illustrated. The optical transmission system 30 includes a primary path 32 and a secondary path 34. The transmission system 30 has a first optical switch 36 coupled at a head end 38 of the transmission system 30 to both the primary path 32 and the secondary path 34. A second optical switch 40 is coupled to both the primary path 32 and the secondary path 34 at an optical node end 42. The first optical switch 36 includes a switch element 44, a primary directional coupler 46, a secondary directional coupler 48, a primary optical detector 50, and a secondary optical detector 52. A coupler 51 is provided to couple the input signal λ1 to primary path 32 when switch 44 is in the upper position. Coupler 51 also couples a portion of the return signal λ2 from path 32 to the primary optical detector 50. A coupler 53 couples the input signal λ1 to secondary path 34 when switch 44 is in the lower position. Coupler 53 also couples a portion of the return signal λ2 from path 34 to the secondary optical detector 52. The second optical switch 40 has a similar configuration, including a switch element 56, four couplers 58, 60, 61, 65 and two detectors 62, 64.
The first optical switch 36 utilizes the directional coupler 46 to couple the input signal λ1 to the primary path 32 or the directional coupler 48 to couple the input signal λ1 to the secondary path 34, depending on the position of switch 44. In the event that the secondary path 34 is in use to carry the input signal λ1 (switch 44 in lower position), a small portion of the input signal λ1 is coupled via path 54 and couplers 46, 51 to the primary path 32. In this way, optical detector 62 at the second optical switch 40 can be used to verify when the primary path 32 has been repaired after a break. If the signal λ1 is detected by the detector 62, then the primary path is successfully communicating the signal, and switch 44 can be switched back to the upper position to couple the full power of input signal λ1 to the primary path.
Similarly, directional coupler 60 of second optical switch 40 couples a small portion of the return signal λ2 to the primary path 32 (via directional coupler 58) for detection by optical detector 50 when the secondary path 34 is being used (switch 56 in lower position). If detector 50 detects the return signal λ2 in the primary path 32 when the switch 56 is in the lower position, this will indicate that the primary path is functioning, and switch 56 can be switched back to the upper position to couple the full power of the return signal λ2 to the primary path.
A break in the primary path 32 will be detected by the detectors 50, 62. In response to such a break, the switch elements 44, 56 are actuated and switch to the secondary path 34. The transmission of the optical signals is thereby maintained in the bi-directional optical transmission system 30. In response to the repair and restoration of the primary path 32, the switch elements 44, 56 are actuated to switch back to the primary path 32. The primary state of transmission is automatically restored without the need to power cycle at the head end 38 or at the optical node end 42.
The bi-directional point-to-point optical transmission system 30 is effective in preventing transmission failure in the event of a single break in the primary path 32. However, by employing two switches and four tap couplers, the system incurs significant link insertion losses (e.g., up to about 6 dB).
What is needed in the art is an optical transmission system that eliminates the losses of the prior art structure while maintaining system reliability.